This invention relates to the use of calcium aluminate and/or alumina bearing clays to enhance and accelerate the reaction rate and to increase early and final strengths in hydraulic-type cements, and also relates to the enhancement and acceleration of the reaction rate in fly ash/cementitious mixtures.
By the term "hydraulic cement", all cementitious water activated cements are intended, including rapid hardening, ordinary, low heat, and sulfate-resistant Portland cements, high alumina cements, conventional hydraulic (i. e., under-water setting) cements, and related products which have been burned or clinkered and then ground to a fine powder.
"Alumina clay" as used herein is intended to encompass any natural or raw finely divided ground clay-like product with maximum particle size not substantially in excess of five microns, and having an alumina component, such as in the form of calcium aluminate or aluminum oxide of at least 35% by weight. As representative, but not intended to be limiting, suitable clays may include kaolin, mullite, crushed bauxite or gibbsite and related unfired bauxitic clays, roller mill clay and similar clay products containing the requisite alumina content, preferably not calcined, and in a finely powdered state.
Calcium aluminate (tricalcium aluminate-3CaOAl.sub.2 O.sub.2) may be a powder or crystals with a specific gravity (25.degree. C.) of 3.038, decomposing at 1535.degree. C., and soluble in acids. Calcium aluminate is an important ingredient of Portland cement. It has been noted over the years that coal fly ash contained amounts of mullite, which is thought to be the fired form of calcium aluminate. This was especially noted in samples of material collected from dust collectors utilized for fly ash. The samples collected from said dust collectors are very fine and have a distinctive yellow clay color.
Cement is manufactured from a number of ingredients, and .lay be considered as a product obtained by intimately mixing together calcareous and argillaceous, or other silica, alumina, and iron oxide bearing materials, burning them at a clinkering temperature, and grinding the resulting clinker. Portland cement is prepared by igniting a mixture of raw materials, one of which is mainly composed of calcium carbonate and the other of aluminum silicates. The most typical materials answering to this description are limestone and clay, both of which occur in nature in a great number of varieties. Marls composed of a mixture of chalk and clay, and shales; are also common raw materials.
Early civilizations utilized lime in various stages of fineness and under a variety of burning conditions for masonry. Volcanic ash pozzolans were used as a source of silica to react with lime to form a cementitious compound that would withstand weathering conditions. Pozzolans are defined as materials which are capable of reacting with lime in the presence of water at ordinary temperatures to produce cementitious compounds. Italian pozzolana (volcanic), trass and santorin earth are examples of naturally occurring pozzolans of volcanic origin.
Artificial pozzolans are prepared by burning at suitable temperatures certain clays, shales, and diatomaceous earths containing a proportion of clay. Diatomaceous silica and some natural amorphous silica deposits may also form pozzolans, either with or without heat treatment. Coal fly ash produced in electric power plants is also termed a pozzolan because the particles react with the lime liberated in the hydration of Portland cement to form a cementitious material.
Based upon study and observations, I believe that the reason fly ash reacts with lime to form cementitious compounds is that the coal fly ash contains alumina in a range from 20-40% in Class F fly ash (ASTM C-618-80). A review of the strength gain curve of fly ash/cement mixtures indicates that strength does not come in until later in the 28-day period or beyond, and then continues to gain strength after this period over mixes containing higher proportion of cement.
For example, it is known that a typical ready-mix concrete using only 400 pounds of cement and 117 pounds of fly ash may show a lower initial strength gain as compared to a mix containing 517 pounds of straight cement, at periods of less than 28 days, but will surpass the strength of 517 pounds of straight cement at the 28-day period. The higher strength gain of the fly ash mixture will remain in effect from there on, and results in a 25% higher ultimate strength while using 117 pounds less cement.
I believe that the initial slow strength gain of the cement/fly ash mix at less than 28 days may be attributable to the fact that the lime is not immediately liberated in the hydration of Portland cement. Also, I believe the fact that alumina in fly ash is enclosed in a glass matrix makes the alumina not readily available to react in the reaction with the lime to form calcium-alumino-silicates and thereby provide early strength. However, over a period of time, the glass matrix is solubilized or eroded, making available its alumina content for reaction.